U.S. patent number 5,226,050 [Application Number 07/646,674] was granted by the patent office on 1993-07-06 for small line width tunable laser.
This patent grant is currently assigned to Lambda Physik Forschungs - und Entwicklungsgesellschaft - GmbH. Invention is credited to Berthold Burghardt.
United States Patent |
5,226,050 |
Burghardt |
July 6, 1993 |
Small line width tunable laser
Abstract
For small line width tuning of a laser, especially a pulsed dye
laser, a reference beam is uncoupled from the laser resonator
(10,18) and directed to a beam position photosensor such as an
adjacent pair of photo diodes. A control signal for synchronizing
the movements of a grating (10) and an etalon (18) of the laser
resonator is derived from a local change of the reference beam in
order that synchronism may be achieved in the movement of both the
etalon and the grating.
Inventors: |
Burghardt; Berthold (Waake,
DE) |
Assignee: |
Lambda Physik Forschungs - und
Entwicklungsgesellschaft - GmbH (Gottingen, DE)
|
Family
ID: |
6398769 |
Appl.
No.: |
07/646,674 |
Filed: |
January 25, 1991 |
Foreign Application Priority Data
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|
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Jan 25, 1990 [DE] |
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4002162 |
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Current U.S.
Class: |
372/20; 372/107;
372/108; 372/32; 372/33; 372/9; 372/98 |
Current CPC
Class: |
H01S
3/213 (20130101); H01S 3/137 (20130101) |
Current International
Class: |
H01S
3/213 (20060101); H01S 3/137 (20060101); H01S
3/13 (20060101); H01S 3/14 (20060101); H01S
003/10 () |
Field of
Search: |
;372/19,20,98,99,107,108,33,9,32 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Konig, et al., Article Published in J. Phys. E: Sci. Instrum. Small
Line th Nanosecond Dye Laser of High Spectral Purity with Double
Functional Grating, 1987. .
Olcay, et al. Article Published in Applied Optics, Tuning of a
Narrow Linewidth Pulsed Dye Laser with a Fabry-Perot and
Diffraction Grating Over a Large Wavelength Range, vol. 24, No. 19,
Oct. 1, 1985..
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Primary Examiner: Scott, Jr.; Leon
Attorney, Agent or Firm: Wallenstein, Wagner & Hattis,
Ltd.
Claims
I claim:
1. A tunable laser comprising:
at least two wavelength selecting members in a given path of a
resonant beam of variable wavelength, said at least two wavelength
selecting members being rotatable to specific wavelength-dependent
angular orientations with respect to said given path to maximize
the intensity of said beam at a chosen wavelength, wherein relative
misorientation of one of said wavelength selecting members with
respect to the other of said wavelength selecting members causes a
divergence of said resonant beam from said given path;
first control means for varying the orientations of said wavelength
selecting members to vary the wavelength of said resonant beam over
a range of wavelength values to a chosen wavelength;
detector means for providing a control signal indicative of said
divergence; and
second control means responsive to said control signal for
operating said first control means to reduce said divergence.
2. The laser of claim 1 wherein said detector means includes
reference beam means for producing a reference beam derived from
said resonant beam and following a reference beam path which varies
in direction from a given reference beam path responsively to said
divergence, and;
optical sensing means disposed in said path of said reference beam
and including means responsive to changes in said direction of said
reference beam for providing said control signal.
3. The laser of claim 2 wherein said first control means includes
first motor drive means for bidirectionally rotatingly driving one
of said wavelength selecting members over a range of angular
positions and second motor drive for bidirectionally rotatingly
driving another of said wavelength selecting members over a range
of angular positions, said optical sensing means includes means
responsive to changes in the direction of said reference beam in a
first direction away from said given reference beam path for
producing an overrotation-indicating signal condition indicative of
overrotation of said one of said wavelength selecting means with
respect to the other of said wavelength selecting means, and means
responsive to changes in the direction of said reference beam away
from said given reference beam path in a second direction different
from said first direction for producing an underrotation-indicating
signal condition indicative of underrotation of said one of said
wavelength selecting members with respect to said other of said
wavelength selecting members, and said second control means
includes means responsive to said overrotation-indicating and
underrotation-indicating signal conditions for controlling said
first motor drive means to move said reference beam towards said
given reference beam path.
4. The laser of claim 3 wherein said optical sensing means includes
first and second photosensing systems disposed to be equally
excited when said reference beam follows said given reference beam
path and to be unequally excited so as to indicate in which
direction said reference beam has moved away from said given
reference beam path, said photosensing systems including means for
producing respectively said overrotation-indicating and
underrotation-indicating signal conditions responsively to
overexcitation and underexcitation of said first photosensing
system with respect to said second photosensing system.
5. The laser of claim 4 wherein one of said wavelength selecting
members is a diffraction grating and the other of said wavelength
selecting members is an etalon.
6. The laser of claim 2 including a beam splitter disposed in said
resonant beam path and configured to split out said reference beam
from said resonant beam.
7. A method for controlling a tunable laser having at least two
wavelength selecting members in a given path of a resonant beam of
variable wavelength, said at least two wavelength selecting members
being rotatable to specific wavelength-dependent angular
orientations with respect to said given resonant beam path to
maximize the intensity of said beam at a chosen wavelength, wherein
relative misorientation of one of said wavelength selecting members
with respect to the other of said wavelength selecting members
causes a divergence of said resonant beam from said given resonant
beam path comprising the steps of:
sensing said divergence of said resonant beam; and
adjusting the orientation of at least one of said wavelength
selecting means responsively to said sensing to restore said
resonant beam to said given resonant beam path.
8. The method of claim 7 including the steps of deriving a
reference beam from said resonant beam so as to deviate from a
given reference beam path responsively to said divergence of said
resonant beam, and adjusting the orientation of said at least one
wavelength selecting members according to the deviation of said
reference beam to restore said reference beam to said given
reference beam path.
9. The method of claim 8 wherein said laser includes a beam
splitter disposed in said resonant beam path and configured to
split out and thus derive said reference beam from said resonant
beam.
Description
FIELD OF THE INVENTION
The instant invention relates to a small line width tunable laser,
especially a small line width tunable dye laser, comprising the
features of the precharacterizing part of claim 1.
BACKGROUND OF THE INVENTION
Such a laser is known from U.S. Pat. No. 4,864,578. That laser,
however, is a continuous mode dye laser having a bandwidth of 1
MHz. The instant invention, on the other hand, is used preferably
with pulsed lasers having a bandwith of 1.2 GHz, in other words
three orders of magnitude bigger.
DD 228 117 A1 discloses a resonator arrangement for a tunable laser
comprising a single reflecting structural element of such design
that good spectral filtering is obtained at high wavelength
selectivity and narrow line width. The tuning of the wavelength is
continuous and effected by means of pressure variation. The laser
beam is expanded by a system of prisms.
A paper by R. Konig, S. Mory, and A. Rosenfeld published in
J.Phys.E.: Sci.Instrum. 20 (1987), pages 200-203.describes a pulsed
dye laser with which the beam is expanded in the resonator by means
of prisms and the wavelength is tuned by turning a grating and/or
an FP etalon.
A pulsed dye laser adapted to be tuned in a wide range of
wavelengths by means of an FP etalon and a grating is known also
from the paper by Olcay, M. R. et al. "Tuning of a narrow linewidth
pulsed dye laser with a Fabry-Perot and diffraction grating over a
large wavelength range" published in Applied Optics, vol.24, 1985,
no. 19, pages 3146-3150.
DE 37 44 323 Al discloses a laser with which stabilization of the
frequency o the laser beam is achieved by inputting part thereof
into a Fabry-Perot interferometer so as to derive an adjustment
signal for the setting of a wavelength selective member. The ring
system of the Fabry-Perot interferometer is imaged on a beam
position photosensor such as an adjacent pair of photo diodes so
that it may be compared with a memorized reference signal so that a
setting signal may be obtained for adjustment of the wavelength
selective member of the laser.
The instant invention starts from a different problem than
stabilizing the frequency of a laser beam.
It is known to narrow down the line or bandwidth of a pulsed laser,
especially a dye laser, by providing an etalon in the laser
resonator in addition to the tuning grating of the laser. The
etalon considerably reduces the bandwidth of the laser beam because
of its selective transmission properties. For tuning such a laser,
the grating and the etalon must be synchronized. These two
wavelength selective members (etalon and grating) must be
positioned with respect to each other in such manner that the
transmission of the etalon will correspond to the wavelength which
is given by the grating. The etalon may be replaced by other
wavelength selective means, such as a Fabry-Perot interferometer,
double refractive crystals, and the like.
When an etalon is applied, the alignment of the etalon with the
grating is accomplished by slightly tilting the etalon with respect
to the laser beam axis.
Now, if the wavelength of the laser is to be tuned (varied) both
the grating and the etalon located inside the resonant cavity
(so-called intracavity etalon) must be tilted in synchronism. It is
the difficulties encountered with such synchronous adjustments of
wavelength selective members (e.g. grating and etalon) that the
instant invention is directed to.
With the state of the art, continuous tuning of the laser
wavelength can be effected only in a very limited range of
approximately 1 nm due to the fact that so-called walk-off losses
caused by the etalon in the resonant cavity occur when the grating
and etalon are moved in synchronism. Furthermore, the minutest
deviations from linearity of the grating and etalon drive will
result in desynchronization so that, in the extreme, the laser will
start to oscillate simultaneously at two adjacent transmission
wavelength of the etalon.
SUMMARY OF THE INVENTION
The invention is aimed at providing a small line width tunable
laser which permits accurate tuning without loss in performance
over a rather wide range of wavelengths.
The laser solving that problem in accordance with the invention is
characterized in the claims.
The invention thus makes it possible to determine any
desynchronization of the two wavelength selective members (e.g.
grating and etalon) so as to obtain a control signal for adjusting
at least one of the two members and apply said signal in feedback
to the drive of that member, thereby assuring the synchronous
tuning of the laser wavelength.
The invention further permits to line up in an easy manner
individual tuning ranges of the laser, especially the dye laser, so
that automatic tuning of the laser wavelength by means of the
etalon is offered over the entire wavelength range of a particular
dye (e.g. 30 nm).
The invention makes use of the finding that desynchronization of
the grating and etalon will lead not only to reduced light
intensity of the laser beam but also result in an ever so small
shift in position of the laser beam in the laser resonator. Such a
change in position and/or direction of the beam can be detected
with the aid of a reference beam, suitably uncoupled, and a control
signal then may be derived therefrom for use in synchronizing the
two wavelength selective members.
BRIEF DESCRIPTION OF THE DRAWING
The invention will be described further, by way of example, with
reference to the accompanying drawings, in which:
FIG. 1 is a diagrammatic presentation of a pulsed laser with an
oscillator and preamplifier, including a computer and separate
control for the dye laser;
FIG. 2 shows details essential to the invention of the dye laser;
and
FIG. 3 is a diagrammatic presentation of another embodiment of a
dye laser which is controlled according to the invention.
DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 illustrates a pulsed dye laser pumped by an excimer laser
and including a resonator composed of a grating 10 and an end
mirror 12. Inside the resonant cavity there is a dye cuvette 14
containing a dye in which a pump in the present case an excimer
laser (not shown) causes population inversion.
The laser moreover comprises a so-called beam expander 16
positioned in the beam path of the resonant cavity to expand the
laser beam, whereby the resolution of the grating 10 can be
enhanced, as is known.
Likewise positioned in the resonant cavity, in a manner known per
se, is an etalon 18 to reduce the bandwidth of the laser beam.
For tuning, i.e. changing the wavelength of the laser, the grating
10 is pivoted in per se known manner by a mechanical control
mechanism (not shown). The wavelength of the reflected radiation is
adjusted by the change of the angle between the laser beam and the
grating. The etalon 18 must be tilted in synchronism with the
grating 10.
FIG. 2 shows essential details needed to do that with the dye laser
according to FIG. 1. Corresponding elements are marked by the same
reference numerals throughout.
The beam expander 16 consists of prisms P1, P2, P3, P4, and P5. A
laser beam reflected by the end mirror 12 is refracted by prism P1
in the direction of prism P2, leaving prism P5 as a substantially
wider beam, as may be seen in FIGS. 1 and 2.
Next to the laser beam passing through prisms P1 to P5, FIGS. 1 and
2 show another beam 20' which is reflected at surface F1 of prism
P1 (FIG. 2) in the direction of the grating 10. It is indeed
reflected at the grating 10, then passes through the dye cuvette 14
in a path which differs from that of the resonant beam oscillating
between the grating 10 and the end mirror 12, and finally is
uncoupled from the resonator as laser beam 20. The uncoupling of
the beam 20 from the resonator corresponds to German patent 29 18
863. Additional stimulation of radiation in the dye cuvette 14 is
provoked where the beam 20 passes through the cuvette. To that end,
the dye molecules in the cuvette are pumped also at the location
where the beam 20 passes through the cuvette.
As shown in FIGS. 1 and 2 (especially FIG. 2), a plate P with
parallel faces is positioned in the resonant beam path between
prisms P1 and P2 to establish synchronization of the etalon 18 with
respect to the grating 10. Part of the oscillating beam reflected
from the grating 10 through the etalon 18 is coupled out at the
plate P with the parallel faces. This uncoupled part beam 25 (FIG.
2) first passes through a prism P6 for compensation of the
expansion and dispersion caused by prism PI. Thereupon the part
beam 25 is deflected at the reflecting base F.sub.2 of a prism P7
in order to be directed as reference beam 24 to a beam position
photosensor such as an adjacent pair of photo diodes 22, such as a
double photodiode or diode array. The reference beam 24, having
been uncoupled from the resonator, is not disturbed by stray laser
light turning back from the final and preamplifier stages of the
dye laser into the resonant cavity. Reference numeral 26 designates
another partial beam 26 which is not of particular interest
here.
When the grating 10 and the etalon 18 are adjusted for tuning of
the wavelength of the laser, desynchronization between grating and
etalon will bring about a shift in the position of the reference
beam 24 with respect to the beam position photosensor 22 and that
can be determined directly there. A corresponding electrical signal
is transmitted from the beam position photosensor 22 through lines
28, 29 to an A/D converter 30 and then input into a computer PC 31
for further processing. The computer PC calculates a control signal
in response to the deviation of the place of the reference beam 24
from a rated position and, based on that control signal, the
tilting angle of the grating 10 or etalon 18 is adjusted so that
the reference beam 24 once more will adopt its desired position.
The deviation of the direction of the reference beam 24 with
respect to its rated position can be determined by means of the
beam position photosensor 22 and provides information both of the
direction of desynchronization (phase) and of the amount of
desynchronization which is evaluated by the computer PC.
A control signal calculated by the computer PC for adjusting the
angle of tilt of the etalon 18 is applied through an interface 33
to the actual control means 32 for the grating and the etalon. Such
mechanical control means are known.
In more detail, it is known in the prior art to control the two
"wavelength selective members" (e.g., the grating 10 and the etalon
18) synchronously by means of step motors 36, 38 (FIG. 2). For
example, when tuning the laser wavelength, both the grating 10 and
the etalon 18 are rotated by ten steps. Such tuning starts from an
ideal alignment of grating and etalon so that the reference beam of
the invention is directed exactly at the middle of the diode pair
22.
The output signals of the diode 22 are transferred to a difference
amplifier (not shown) at the input of the A/D converter 30. If the
laser beam is centered exactly between the elements of the diode
pair, the signals on lines 28,29 will be equal, and the output
signal of the difference amplifier will be zero. If the laser beam
shifts due to a misalignment of the grating and the etalon, the
output signal of the difference amplifier will no longer be zero.
The direction of the shifting is reflected in the sign of the
output signal of the difference amplifier.
Experimentally, the direction of misalignment, i.e., where the
grating or the etalon is rotated too much, can be related to the
sign of the output signal of the difference amplifier. Also, the
amplitude of the output signal of the difference amplifier reflects
the magnitude of the misalignment. For a given laser system, these
dependencies must be determined experimentally, since the magnitude
of the output signal of the difference amplifier depends also on
the position of the diodes with respect to the reference beam
24.
For example, if normally each tuning step of the laser includes a
rotation of both the grating and the etalon by ten steps, and if a
misalignment is evidenced from a difference in the output signal in
the difference amplifier, and having a certain magnitude and sign,
this misalignment is compensated for by rotating the grating only
nine steps, whereas the etalon is still rotated by ten steps. A
correct synchronization of the grating and the etalon is reached
when the output signal of the difference amplifier is zero again.
In addition to using a diode pair as a position sensor 22, it will
be evident to those skilled in the art that a variety of such
sensors may be employed, such as charge coupled device arrays or
the like.
FIG. 3 presents a modification of the embodiment of laser control
shown in FIGS. 1 and 2. This modified embodiment does not comprise
a separate computer PC. Instead, the measurement signal arriving on
lines 28,29 from the beam position detector 22 (and being a measure
of desynchronization) is entered directly into the integrated laser
control means 32 through and A/D converter 30. The control signal
for synchronizing the etalon 18 and the grating 10 is inputted
directly via a line 34.
While the invention has been described with reference to a
preferred embodiment, it will be understood by those skilled in the
art that various changes may be made and equivalents may be
substituted for elements thereof without departing from the broader
aspects of the invention. Also, it is intended that broad claims
not specifying details of a particular embodiment disclosed herein
as the best mode contemplated for carrying out the invention should
not be limited to such details.
* * * * *